The invention relates to a pressing-in unit for a pressure diecasting machine, particularly for a hot-chamber pressure diecasting machine for processing molten metals, having a casting plunger for pressing the casting material into a mold, which casting plunger can be acted upon by way of a pusher rod which is connected with a linear drive driven by an electric motor, which linear drive, after the filling phase of the mold, is held in a pressure phase for achieving a pressure in the casting material, a spring element being connected between the casting plunger and the pusher rod.
From European Patent Document EP 0 430 616 A1, a pressing-in unit is known, in which a spindle is provided as a linear drive for driving a nut. The drive of the casting plunger connected with the linear drive takes place by way of a belt driven by an electric motor. In this case, the belt drive acts upon the spindle by way of an electromagnetically controllable disk coupling so that, in this manner, the advancing speed of the casting plunger and, after the filing of the mold, the pressure to be maintained in the casting material can be controlled. A rotational-speed-dependent signal, which is emitted by a speedometer connected with the spindle drive, is used for the control. A drive of this type for the pressing-in unit requires relatively high expenditures. Mainly because of the susceptibility of such couplings to wear, the controlling and regulating of the disk coupling has disadvantages.
From the Patent Abstract of Japanese Patent 0 7155 925, a pressing-in unit of the initially mentioned type for a pressure diecasting machine is known, in which an elastic spring element is connected between the casting plunger and the pusher rod and avoids undesirable pressure peaks which, as a result of the system, arise during the stopping of the drive and during the transition to the regulating of the pressure because of the forces of inertia inherent to the drive. While such problems do not occur in the case of plastic injection molding machines because of the elastic behavior of liquid plastic masses, the conditions in the case of pressure casting machines for processing molten metal are different in that the molten metal can virtually not be compressed.
In the case of injection molding machines for the processing of liquid plastic masses, it is also known to control the extruder spindles provided there for the pressing-out by means of electronic regulators such that, in the critical phases of the ram pressure buildup during the melting as well as when maintaining the pressure in the after-pressure phase, the desired advancing rates or torques can be exercised for maintaining the pressure (German Patent Document DE 43 45 034 A1). Information is also supplied there that a similar driving principle can also be used in the case of pressure casting machines if an elastic element is connected between the drive and the movable injection elements.
It is an object of the present invention to construct a pressing-in unit of the initially mentioned type in a manner which is as simple as possible.
In order to obtain a simple type of construction, it is provided in the case of a pressing-in unit of the initially mentioned type according to the invention that the spring element is constructed as an elastic plastic component or as a liquid spring and is designed such that the reaction force onto the casting plunger resulting from its prestressing will be high enough for applying the axial force to the casting plunger which is required for achieving the required pressure in the casting material. Such a spring element can absorb the moment of inertia occurring during the braking of the drive. The resulting spring travel prevents the further displacement of the casting plunger and thus also the occurrence of pressure peaks without the requirement of complicated control measures.
Thus, with respect to a pressing-in unit of the initially mentioned type, two objects are achieved according to the invention. On the one hand, the moments of inertia caused during the braking of the drive by the masses of the electric motor and of the transmission can be absorbed. Thus, as of the point in time of the switch-over from filling the mold to pressure, the casting plunger is no longer advanced. On the other hand, the additional travel occurring as the result of the moments of inertia is used for prestressing the spring element and the then generated prestress is utilized for acting upon the casting plunger by means of an axial force which is high enough for maintaining the desired after-pressure in the casting material.
These measures simplify the controlling of the casting plunger. The reason is that the regulating of the after-pressure can be reduced in this manner to a secondary speed control in a regulator cascade with a primary automatic power control.
As a further development of the invention, a sensor, which is connected with the control of the drive, can be assigned to the spring element. This sensor detects the axial force exercised by the spring element and correspondingly influences the drive control. As a result, the electric drive can be controllable such that the spring element is not compressed beyond a predefined extent.
As a further development of the invention, an elastic plastic ring can be provided as a spring element which is held between two flanges, of which one is held on the pusher rod and the other is fixedly connected with a sleeve telescopically guided on the pusher rod. The relative movement between the pusher rod and the sleeve compresses the plastic ring, specifically by an extent which corresponds to the adjusting path of the pusher rod caused by the after-running of the drive after the braking operation. It was found that such a plastic ring, particularly if it consists of a coated polyurethane caoutchouc, can apply the high forces occurring in the case of pressure diecasting machines for the compression of the molten metal. This plastic ring can also be constructed of two or several parts which can be deformed successively, so that different spring characteristics can also be implemented for the absorption of the moment of inertia and then for the application of the axial pressure force.
In a constructively simple manner, the pusher rod may have an extension of a smaller diameter which penetrates the plastic ring and on which the sleeve is also carried. The step provided between the extension and the pusher rod can then be used as a bearing for the disk held on the pusher rod and for a pressure sensor assigned to this disk. It is also conceivable in this case that the spring element and the arrangement of the sleeve are coordinated with one another such that the extension can be adjusted only a certain path relative to the sleeve.
As in the prior art, a threaded drive can be provided in a simple manner as the linear drive which consists of a spindle and a nut guided thereon. However, it is also conceivable to provide a rack-and-pinion drive as the linear drive which permits a robust construction and also has a low noise development. The rack-and-pinion drive can take place by way of a transmission which is applied to both sides of the rack, so that no one-sided stressing of the rack occurs. In the case of such a method of construction, two motors can also be provided for driving the transmission so that a power adaptation with higher dynamics as well as a higher acceleration and deceleration becomes conceivable. The transmission may be completely encapsulated, so that no oil outflow is possible in the direction of the molten bath.
However, the linear drive can finally also be a carriage guide driven by a connecting rod of a crank mechanism, in which case, if a certain vertical adjustment is provided, an optimal adaptation of the power course is conceivable to the mold filling stroke.
However, a liquid spring, which is known per se, can also be provided as the spring element (xe2x80x9cLUEGER Lexikon der Technikxe2x80x9d, Volume 12xe2x80x94Automotive Engineeringxe2x80x94Edition 1967, Verlag DVA Stuttgart, Page 223). At the pressing-in forces in the order of several tons occurring in the case of pressure diecasting machines, such cylinder/piston units, which as a rule are filled with oil, can provide the required spring travel, in which case, the spring force can then also here be utilized for acting upon the casting plunger.
In a practical embodiment, the liquid spring can be provided with an immersion piston which, by the force of a spring, is pressed into the interior of the cylinder in order to subject the liquid to a certain prestress even before it is acted upon by the pusher rod. As a further development of the invention, the cylinder of the liquid spring can also be provided with an opening to which an excess pressure limiting valve and a pump are connected for the possible return of exited liquid. A compressive strain converter can be connected to the connection line to the pressure limiting valve, which converter can, in turn, be connected with the control of an electric-motor drive.
However, additional liquid volumes can advantageously also be connected to the connecting line. The controllable connection of the liquid volume contributes to the change of the spring characteristic. Also when a liquid spring is used, an adaptation of the spring characteristic can also be achieved in this manner which is similar to that of a multipart plastic ring described above.
The invention is illustrated in the drawing by means of embodiments and will be explained in the following.